Integrate API Payment Gateways with USDC

The skill teaches how to quickly add x402 payment walls to Node.js/Express applications and servers, monetizing each API call by requiring USDC micropayments over Base L2. Installation pulls the wrapper and ethers from NPM (or references a local x402-express folder from the M2MCent repository if not yet published):

npm install x402-express ethers

Implementation is a single middleware import: instantiate X402Wrapper with an RPC URL (https://mainnet.base.org), a RELAYER_PRIVATE_KEY (required, needs Base L2 gas), and a recipient wallet address (required, the developer's net payout address), then apply x402.requirePayment(amountRaw) to any Express route - amountRaw is USDC with 6 decimals, so "20000" means $0.02. As of v1.1+ the escrow contract address is hardcoded inside the wrapper and cannot be overridden; passing an external contract address throws an error. When the middleware lets a request through, payment has already settled and the response can include the transaction receipt.

On the agentic side, client agents are expected to send a base64-encoded JSON payload in a Payment-Signature header structured as { from, validAfter, validBefore, nonce, signature }. Settlement is immediate and atomic: the wrapper itself acts as the relayer, meaning the web server calls M2MCentEscrow.settle() on-chain directly, so the RELAYER_PRIVATE_KEY account must hold ETH for gas - the client consuming the API pays zero gas. When building a new micro-SaaS on this pattern, the developer's .env variables must match what the wrapper expects.

Use it when asked to add a payment barrier to an API, monetize an MCP server that was just built, or require USDC micropayments to use an endpoint. It requires valid environment variables (RELAYER_PRIVATE_KEY, MY_WALLET_ADDRESS) and a funded gas balance on Base L2 to settle payments. It covers only the x402 wrapper/middleware itself, not full production infrastructure hardening or key management, and it targets Node.js/Express specifically - other runtimes or frameworks need additional adaptation.

Input is an Express route to protect and a USDC fee amount in 6-decimal raw units, plus the relayer's private key and payout wallet address as environment variables. Output is a paywalled endpoint that only responds once an incoming Payment-Signature header has been verified and settled on-chain via M2MCentEscrow.settle(), with the settlement transaction available on the request object.

Developers or agents building a micro-SaaS or monetized API/MCP server on Node.js/Express who want USDC-denominated, per-call payment gating on Base L2 without building the payment-verification and on-chain settlement logic themselves.